The present invention relates generally to a light-emitting diode (LED) and, more particularly, to an LED with an omni-directional reflector for enhanced light extraction efficiency.
LEDs emit light in one or more of the infrared, visible, and ultraviolet spectral regions when an electrical current is passed through a semiconducting, light-emitting region. As shown in FIG. 1, a common LED 1 emits light in the 550 nm-700 nm wavelength range. LED 1 comprises an aluminum gallium indium phosphide (AlGaInP) active region 2 lattice-matched to a gallium arsenide (GaAs) substrate 3. Active region 2 comprises a light-emitting region 4 surrounded by two oppositely doped confinement layers 5. LED 1 may be referred to as an absorbing-substrate light-emitting diode (AS-LED) due to the light-absorbing characteristic of the GaAs substrate 3. The chemical formula for the composition of the active region material is (AlxGa1-x)0.5In0.5P, where x can vary between 0.0 and 1.0. This chemical composition ensures that the (AlxGa1-x)0.5In0.5P, commonly abbreviated as AlGaInP, is lattice-matched to the GaAs substrate 3.
Typically, AS-LED 1 comprises a window 6 overlying active region 2. Window 6 may be composed of gallium phosphide (GaP) that may also contain small amounts of other elements such as Al and In. Window 6 may also be composed of aluminum gallium arsenide, or AlxGa1-xAs, commonly abbreviated as AlGaAs. An optically opaque top contact 9, typically comprising a highly electrically conductive metal or alloy is formed over window 6, and a highly electrically conductive substrate contact 10 is formed adjacent substrate 3 opposite active region 2. Window 6 may also be referred to as a current-spreading layer, because window 6 distributes electrical current over a larger area than that covered by top contact 9, as shown in FIG. 2. The active region 2, that comprises the light-emitting region 4, may be a double heterostructure (DH) or, more commonly, a multiple quantum well (MQW) structure as is known in the art.
When current passes between top contact 9 and substrate contact 10 through active region 2, light is emitted from light-emitting region 4 in all directionsxe2x80x94as illustrated by the dashed light-emission profile 7 of FIG. 2. Light-emission profile 7 corresponds to the current concentration in light-emitting region 4. Light emitted toward substrate 3 is absorbed by GaAs substrate 3. Light emitted away from substrate 3 and having an incident angle approaching normal or being normal to the top or bottom surfaces of window 6 is emitted from LED 1. Light having an oblique incident angle to window 6, however, may be reflected at the top surface of the window and subsequently absorbed by substrate 3.
In an effort to improve the light-extraction efficiency of AS-LED 1, a distributed Bragg reflector (DBR) 8 may be disposed between active region 2 and substrate 3. DBR 8 is only partially reflective, however, with on-resonance wavelengths and normal incidence angles providing the highest reflectivity. The light not reflected by DBR 8 will be absorbed by GaAs substrate 3.
FIGS. 3-7 illustrate another LED structure. Because this structure is formed with a transparent substrate 13, LED 11 is referred to as a TS-LED 11. An active region 12 is formed on a GaAs substrate 13a (similarly to AS-LED 1). Then, a GaP or AlGaAs window 16 is formed over active region 12, and GaAs substrate 13a is removed from the structure. Next, active region 12 and GaP or AlGaAs window 16 are wafer bonded to a transparent GaP substrate 13. Light emitted from active region 12 toward transparent GaP substrate 13 passes through transparent GaP substrate 13 without being absorbed and may escape from the GaP substrate 13 or be reflected by the device packaging (not shown).
Although TS-LED 11 provides better light extraction efficiency than AS-LED 1, there are several disadvantages associated with TS-LED 11. The semiconductor-to-semiconductor wafer bond between active region 12 and transparent GaP substrate 13 requires high precision and is extremely sensitive to contamination, thus the processing costs are high and the process yield is low. Another disadvantage of TS-LED 11 is that transparent GaP substrate 13 is expensive. In addition, the GaP/AlGaInP interface and the GaP substrate produce a higher forward voltage as compared to AS-LED 1. The higher forward voltage reduces the efficiency of the TS-LED 11.
Therefore, a need exists for an LED which provides high light extraction efficiency without the disadvantageous expense, low yield, and forward voltage of a TS-LED.
To meet this and other needs, and in view of its purposes, an exemplary embodiment of the present invention provides a high extraction efficiency light-emitting diode having a reflective submount. A light-emitting region is disposed between a top contact and a conductive holder and extends beyond an area underlying the top contact. An omni-directional reflector is disposed between the active region and the conductive holder. According to one embodiment, the reflector comprises one or more electrically conductive contacts configured to correspond to an area beyond the area underlying the top contact. According to one embodiment, the reflector comprises a dielectric layer having a refractive index of between about 1.10 and 2.25, contacts extending through the dielectric layer, and a reflective conductive film composed of a metal.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. The invention will be described in the context of the AlGaInP material system. However, it is noted that the invention can also be reduced to practice in LEDs composed of other materials, in particular in LEDs with light-emitting regions composed of GaAs, AlGaAs, GaN, GaInN, AlGaN, and AlGaInN.